Portable electronic and electrical devices (PEDs) require lightweight and compact power sources. While some very low power devices (e.g. calculators) can be powered directly from energy sources such as photovoltaic cells (PV), for many practical PEDs a higher capacity energy storage device such as an electro-chemical battery (battery) is required. Many PEDs employ rechargeable batteries as a means for storing energy during a recharging process and then release the stored energy while the PED is in use.
As is well known to those skilled in the art, electro-chemical batteries have specific requirements for safe and efficient charging. These specific requirements vary by the specific battery technology but in all cases, safe and efficient charging requires applying energy within specific voltage and current levels for specific periods of time. For this reason, the typical energy source employed for charging batteries is generally from stationary and much larger source of energy such as, for example, electric utility mains power or an automobile battery.
The need to have access to a large power source at a fixed stationary location is often not convenient since the very nature of a PED is that the PED is portable and often in use some distance away from a large stationary source of power to recharge the battery. As is also well known to those skilled in the art, there are many potential sources of energy that might be employed to recharge a battery. However, many of these alternatives to large fixed sources of energy are often of an intermittent nature and may be of too low a level of power to be transformed by conventional methods to suit the specific voltage and current levels needed to safely and efficiently charge a battery. Examples of such potential alternative sources of energy are photovoltaic cells, manually operated electro-magnetic mechanical generators, and even brief connections to electric power utility mains (whenever a user can briefly pause near such mains).
Various simple means to recharge an electro-chemical battery using intermittent and variable sources such as photovoltaic cells are well-known in the prior art. An example of such a system is found in U.S. Pat. No. 3,921,041 entitled “Charging Circuit for Battery-Operated Devices Powered by Solar Cells”, issued to Geoffrey Mellors, et al. on Nov. 18, 1975. FIG. 1 depicts a circuit employed by Mellors in this patent.
In the system of FIG. 1, the battery 102 can only be effectively charged when there is sufficient light intensity such that the voltage output from the PV cell 106 exceeds the battery voltage plus the forward biased diode threshold of device 104. In this case, any energy generated by the PV that is below voltage and current threshold is wasted since it cannot charge the battery B1.
A further problem with the approach of FIG. 1 is that some modern battery technologies such as Li-Ion cannot be safety charged without charge management electronics to limit voltage and current levels into the battery B1. Such charge management electronics may have even more demanding voltage and current levels than charging the battery directly. For this reason it is not uncommon to employ charge management electronics between the PV cell and the battery. An example of such a system is illustrated in FIG. 2. In this figure a high efficiency electronic circuit, typically in the form of a DC to DC converter 204 and associated regulation and sensing circuitry (not shown), is used to convert the DC voltage from the array of PV cells 202 to a voltage more suitable than a direct connection between the PV cells and the battery as in FIG. 1.
While the approach in FIG. 2 is an improvement over that of FIG. 1, it still suffers from the inability to effectively make use of low levels or brief periods of illumination for two related reasons: 1) If the level of illumination is of a low level the voltage generated by the PV cell will not be sufficient to operate the DC to DC converter as any practical electronic device requires voltages well above zero to operate with an efficiency. And 2) If there are very high levels of illumination for a brief period, the PV cells might produce more energy than the batteries can safety absorb during the brief period of illumination. In this case the excess energy generated by the PV cells will be wasted since it cannot be captured and stored in the battery.
U.S. Pat. No. 3,921,049 entitled “Battery-Less Solar Power System” and issued to Miguel Timm on Apr. 9, 2002 recognizes the value of capturing variable and intermittent energy such as from a PV cell source and accumulates this energy in a capacitor. However, this invention employs the stored energy to operate a device directly. When this stored energy storage is sufficiently depleted, the device ceases operation. Consequently, its use is limited to specific applications where such interruptions are allowable.
There is a need in the prior art to provide the means for the efficient capture, accumulation and use of potential alternative sources of energy whose supply may be intermittent and variable in magnitude. Such alternative sources of energy include photovoltaic cells, manually operated electro-magnetic mechanical generators, wind power, and wave power. Further, there is a need to provide an effective means to use this captured energy to properly and efficiently recharge the battery power source(s) of an electrical device.